EVANS, Carol; DICKINSON, Amanda; CROLL, Roger; Dalhousie University; Whitehead Institution, MIT, ; Dalhousie University: Development and Fate of Muscles in Larvae of the Marine Snail Ilyanassa obsoleta

Ilyanassa larval provide a simple model to investigate muscle development in molluscs. Phalloidin binding to F-actin reveals that the larval retractor muscle (LRM) is the first to develop in the embryo. During the trochophore stage (2.5-4 days) fine fibers can first be detected along the right side of the animal. At the early veliger stage (7-10 days) the LRM branches ventrally into the head and body and dorsally into the velum�an anterior structure used for feeding and swimming. Growth of the pedal retractor muscle (PRM) differs from the LRM in that few fibers are present by the early veliger stage. However, by late veliger stage the PRM becomes extensive, sending the majority of fibers anteriorly and then ventrally into the foot. Besides their morphology and development, the LRM and PRM also differ in their fate. At the time of metamorphosis the LRM is first seen to deattachment from the shell. The fibers then become wavy in appearance and begin to degenerate into blebs containing actin and eventually disappear altogether. The PRM differs from LRM in that the fibers are present through adult stages. Studies in other gastropod species describe similar degeneration of the musculature as part of metamorphosis. Emerging studies in molluscs suggest that apoptosis may be one factor that contributes to the degeneration of the LRM. Preliminary data indicate muscle cells positive for TUNEL staining. The latter procedure alongside nuclear stains, Hoescht and propidium iodide will be used to test the hypothesis that LRM degenerates by apoptosis. Currently, methods are also being explored to study the role of caspase activity in metamorphosis. Additional study is needed to determine the other cellular events that define metamorphosis.